The present invention relates to an optical disc, a direct stamper to be used in molding optical discs, and a method for manufacturing the direct stamper.
Conventionally, stampers for manufacturing optical discs have been manufactured by so-called mastering process. This manufacturing process generally employs a technique so called photolithography, starting with the formation of a pits-and-bumps pattern of photoresist corresponding to signals on a glass disc. A positive-type photoresist layer on a glass disc is exposed spirally to a laser beam whose intensity is modulated in response to a signal which is to be recorded, and then subjected to development, by which pits of the resist are formed in track shapes. Thereafter, an electrically conductive film is formed on the surface, and a nickel thick film is formed by electroforming further thereon. The thick film, which is about 0.3 mm thick, is stripped from the original glass disc, by which a stamper is obtained. In this nickel disc, bumps to which pits of the photoresist have been transferred are formed in a spiral shape. By injection molding with this disc, an optical disc having bit strings containing information is made up.
The process for making this stamper includes 10 or more steps, taking much time and cost for manufacture. Further, such large numbers of steps would incur not a few defects due to dust or human errors or the like, resulting in lowered yields.
In recent years, it has been being attempted to make up a stamper without requiring such many steps but with fewer steps. Japanese Patent No. 2765421 discloses a manufacturing method which includes the steps of providing a crosslinkable inorganic or organic layer on a substrate, subjecting the substrate to irradiation of a laser beam that has been intensity-modulated with a recording signal and further to development so that bumps corresponding to a signal pattern are provided on the substrate, and further strengthening the bumps by heating, where the resulting substrate is then put into direct use as a stamper. The prior art is explained by referring to FIGS. 4A-4D. FIG. 4A shows a nickel substrate 1 and a crosslinkable inorganic or crosslinkable organic substance layer 2. FIG. 4B shows a state that the crosslinkable substance 2 is exposed to a signal-modulated laser beam 3 that has been converged by a recording lens 4. FIG. 4C shows a state that exposed portions as a latent image of laser exposure are locally heated, thereby crosslinked, to form marks 5. Through development of this, unexposed portions are dissolved, while the exposed portions alone remain as 6. The crosslinking substance 2 of these has an action of so-called negative-type resist. As used herein, the term “crosslinking substance” refers to a crosslinkable substance that has been at least partially crosslinked. FIG. 4D represents the substrate in the finished state. When this substrate is subjected to hard baking process at a high temperature of 300° C., the mark portions 6 are promoted to be further crosslinked, and thereby strengthened. The substrate made in this way is put into direct use as a stamper. Therefore, this stamper is called direct stamper.
Also, Unexamined Japanese Patent Publication No. 7-326077 discloses a method which includes the steps of forming bumps of photoresist (crosslinking substance) on a substrate, and subjecting the substrate to high-temperature heating to accelerate its crosslinking, where the resulting substrate is then directly used as a stamper. The method of this patent publication differs from that of Japanese Patent No. 2765421 in the following points. That is, photoresist is selectively exposed to light so as to generate an acid. Thereafter, heating the whole substrate causes the acid to act as a catalyst, which causes crosslinking to occur to the exposed portions alone. Next, subjecting the whole substrate to exposure to light causes an acid to be generated at portions which have not been exposed at the preceding time, and the unexposed portions, to which crosslinking has not occurred, are dissolved by the developer. Thus, the portions that have first been exposed to light remain as bumps.
However, prior-art direct stampers are in either case such that bumps of organic or inorganic photoresist (crosslinking substance) are formed directly on a nickel substrate. Because of this, although the photoresist is enhanced in hardness by high-temperature baking, yet the bonding strength between the photoresist and metal is not so high inherently. Also, in the case of DVDs, photoresist bumps are 0.3 μm wide and, at the shortest, 0.4 μm long, hence very small in bonding area.
Each of these direct stampers, when set to a mold of a molding machine, forms a cavity with a mirror-surface mold. High-temperature resin is injected thereinto at high pressure. In this operation, the high-pressure resin exerts an action of stripping photoresist bumps off.
Whereas the mold temperature is generally set at around 100° C., the resin to be injected is at a high temperature of about 300° C. or more. Thermal expansion and contraction are repeated, where the surface temperature of the direct stamper is elevated by resin injection and returned to the original temperature by extraction of a molded disc (optical disc). There is a large difference in coefficient of thermal expansion between nickel or other like metals and photoresist, and stress due to the difference in coefficient of thermal expansion repeatedly acts on the photoresist bumps so that the photoresist bumps might be finally stripped from the substrate.
In molding of DVDs (optical discs) with the prior-art direct stamper, regenerative signal errors of the DVDs due to losses of photoresist bumps would reach a permissible limit value at around 5000 shots of molding. In the case of DVDs, an error due to defects is called PI error, which is determined to be 280 or less in 8ECC blocks before error correction. Because of this, the molding of DVDs (optical discs) with the prior-art direct stamper has been incapable of molding of about 5000 shots or more.